This invention relates generally to production of superconductors, i e., products having extremely low resistance to flow of electrical current, and processes for the production of such products. Following the initial discovery of high transition temperatures (T.sub.c) in the K.sub.2 NiF.sub.4 -type compound La.sub.2-x Ba.sub.x CuO.sub.4, research activity upon superconducting oxides has been focused on the compound YBa.sub.2 Cu.sub.3 O.sub.7-x and its isostructural rare earth analogue. After the initial attainment of superconductivity at 95K, there have been reports published in the scientific literature of evidence for T.sub.c 's as high as 240K. Among all of these materials there are some common structural features; namely, the presence of CuO layers and a significant degree of antisotrophy due to an elongated c-axis in the orthorhombic unit cell. Early band structure work on compounds with the K.sub.2 NiF.sub.4 -type structure emphasized this reduced dimensiorality and the impact of these crystallographic features on the superconducting properties.
Practical application of these remarkable materials requires control of both the metallurgical and ceramic processing techniques to attain optimal critical parameters, such as the critical current density (J.sub.c) and upper critical magnetic field (H.sub.c2), while preserving the high superconducting critical temperature (T.sub.c &lt;90K). Technologically, it is important to maximize these parameters at the boiling point of liquid nitrogen (77K) or above in order to utilize this convenient, inexpensive cryogenic liquid.
It is recognized that optimal T.sub.c values and homogeneity of the superconducting phase, as evidenced by narrow transition widths, are exceptionally sensitive to the oxygen concentration in the orthorhombic phase of YBa.sub.2 Cu.sub.3 O.sub.7-x and its rare earth analogues. The best, consistent results occur for X in the range 0.1 to 0.2. This fact is of importance to all processing of bulk material.
While the potential technological impact of these materials is enormous, their immediate application is severely limited by certain inherent drawbacks. In their present state, the ceramic super-conductors are brittle and unable to support any significant stress In addition, they are environmentally unstable, as they easily react with atmospheric moisture/CO.sub.2 and are decomposed by exposure to high temperatures. Furthermore, critical current densities achievable in present bulk materials are on the order of 10.sup.3 amp/cm.sup.2 at zero field, whereas many of the applications envisaged require values over 10.sup.5 amp/cm.sub.2, at quite high magnetic fields (&gt;10T).
The bulk of the ongoing research on high T.sub.c superconductors is focused on the "123" compounds more specifically YBa.sub.2 Cu.sub.3 O.sub.7-x, where 0.0&lt;.times.&lt;0.5. The superconducting form of this material (T.sub.c .times.94K) has an orthorhombic structure based on a stacking of three perovskite-like unit cells, (BaCuO.sub.2.5):(YCuO.sub.2.5) with rows of oxygen vacancies on the {001} type planes. The critical temperature appears to be very sensitive to the oxygen content, which in turn reflects the ionization state of the three Cu ions. When.times.0.5, all the copper is present as Cu.sup.2+, where at x=0.0 there is one Cu.sup.3+ per unit cell (i.e. per molecule of YBa.sub.2 Cu.sub.3 O.sub.7-x). Superconductivity requires an oxygen content of about O.sub.6.9.
It is clear that oxygen control during processing is critical to successful superconductor fabrication. In the conventional processing approach, the YBa.sub.2 Cu.sub.3 O.sub.7-x shapes are produced by mixing and pressing the individual oxides, in powder form, and which are then reacted/sintered, typically at 900-950.degree. C. in air for 12-16 hours.
The orthorhombic phase loses oxygen and transforms to a non-superconducting tetragonal structure at high temperatures, and accordingly, the sintered product is slowly cooled to allow oxygen pickup and reversion to the superconducting form.
Product densities from 50%-90% of the theoretical value (6.4 Mg/m.sup.3) have been achieved by such conventional processing, the higher values ascribed to liquid phase sintering as YBa.sub.2 Cu.sub.3 O.sub.7-x CuO above 950.degree. C. The porosity left in these compacts not only diminishes their structural integrity and increases the surface area exposed to environmental attach but is believed to play an important role in reducing critical current densities. More importantly, the oxygen content varies during the process and within the final product, making it difficult to achieve homogeneity in composition/microstructure/properties. There is, therefore, need for a process which achieves significant improvements, as are characterized by the herein disclosed process, and including rapid densification of a powder preform close to 100% of theoretical, while retaining superconductivity, as will be described, and while minimizing micro-structural degradation.
Also, there is need for processing the preform and product in such a way as to achieve relatively large weights and sizes.